A steam turbine has a defined steam path which includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. Some areas in a steam turbine may become stagnant with respect to steam flow. For example, there may be insufficient driving force to provide sufficient cooling steam flow in all areas of the turbine. As a result, the area in which steam flow is stagnant may have an increased temperature.
In steam turbines, it is common to design the first stage with negative root reaction to provide steam cooling for the first wheel. The negative root reaction generates some leakage through the first stage wheel holes. In a double flow steam turbine, it is possible to have both turbine ends designed with negative root reaction. However, it is expected that some steam will be trapped in the tub region. The tub region in a double flow steam turbine may therefore become stagnant with respect to steam flow. If both ends have the same root reaction or slightly different root reactions, there will be insufficient driving force to have cross flow from one end to the other. As a result, a high temperature may occur in the tub region due to windage heating of the stagnant steam. This high temperature potentially shortens the useful life of the rotor and may lead to failure of the steam turbine.